#include <bits/stdc++.h>
using namespace std;
#define sz(v) int(v.size())
#define ar array
typedef long long ll;
const int N = 5e3+10, MOD = 1e9+7;
int n, K, small[N];
vector<int> one, two;
int dp[N][N], cost[N][N];
void solve() {
cin >> n >> K;
for (int i = 0; i < 2 * n; i++) {
char c; cin >> c;
if (c == 'A') one.push_back(i);
else two.push_back(i);
}
// for (int x : one) cerr << x << ' '; cerr << endl;
// for (int x : two) cerr << x << ' '; cerr << endl;
for (int i = 0; i < n; i++) {
small[i] = lower_bound(two.begin(), two.end(), one[i]) - two.begin();
}
for (int l = 0; l < n; l++) {
int cur = 0;
for (int r = l; r < n; r++) {
cur += max(small[r] - l, 0);
cost[l][r] = cur;
}
}
for (int i = 0; i < n; i++) for (int j = 0; j <= K; j++) dp[i][j] = MOD;
for (int i = 0; i < n; i++) {
for (int l = i; l >= 0; l--) {
for (int k = 1; k <= K; k++) {
int prv = MOD;
if (l == 0) prv = (k == 1 ? 0 : MOD);
else prv = dp[l-1][k-1];
dp[i][k] = min(dp[i][k], cost[l][i] + prv);
}
}
}
cout << dp[n-1][K] << '\n';
}
int main() {
ios::sync_with_stdio(false); cin.tie(0);
int T = 1;
// cin >> T;
while (T--) solve();
}
